In troubleshooting an unrelated problem with Firebug, I took a look at the amount of http calls to static content, and it seems fairly significant. What are the recommended approaches supported by Splunk to cache content?
When I have done Plone development in the past I have used Varnish  and that had a dramatic impact on a heavy UI. I am curious what caching system splunk customers use. We are currently using ngnix, so perhaps enabling their caching might be a good choice .
I've used Apache httpd in front of Splunk with mod_cache and mod_rewrite/mod_expires to cache static content both on the client side (Expires header) and on the reverse proxy (in-memory cache). This brought a noticeable performance gain in some cases. This is possible with nginx as well. If you need to optimize the UI performance, you could cache all URIs that start with either /<locale>/static or /<locale>/modules.
eg. /en-US/static/* and /en-US/modules/*
The only thing that you might be aware of is that some custom modules from splunkbase are generating static content on-the-fly. This generated content should not be cached and those modules might not work correctly with such a configuration.
As an example, the nginx configuration could look like this (not tested):